What language does IntelliJ's HTTP client's scripts use?

What language does IntelliJ's HTTP client use? I thought it's some sort of JavaScript, but then discovered that strings here don't have `Includes()`
```http

GET http://localhost:8080/api/v1/hello-world
Authorization: bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiJkb25hbGRfZCIsInJvbGVzIjpbInVzZXIiLCJhZG1pbiJdLCJpYXQiOjE1MTYyMzkwMjJ9.DFVsneBfFOpClS3kwHL58RpPzW06vQxmn0krlQd_IZc

> {%
    client.test("GET /api/v1/hello-world: valid token", function() {
        client.assert(response.status === 200, "Response status is not 200");
        client.test(response.body.includes())
    });
%}
```